Ball hitting sound and ball hitting feeling are major factors of the properties of golf clubs. For an iron club head, a head manufactured by hot forging of low-carbon steel, that is, manufactured by forging so called soft iron is regarded to provide good ball hitting sound and ball hitting feeling. The main two types of the shape of the iron club head are a muscle back type in which mass is not distributed to the periphery and a cavity back type in which mass is distributed to the periphery and a cavity is formed on the rear surface side of the face to increase the sweat area.
It is generally regarded that a muscle back type is superior than a cavity back type as for hitting sound and hitting feeling. This is because it is said that robust structure (thickness), manufacturing method (fining), softness of material are mixed to generate an effect that a soft feeling can be obtained while providing a stable response. Mass is distributed to the periphery of the head in a cavity back type, so that the thickness of the face is relatively thinner than that of a muscle back type.
Accordingly, it is said that a cavity back type does not provide good ball hitting feeling relatively. Since it is also said that the sweet area of a muscle back type is smaller than that of a cavity back type, a muscle back type is considered to be favorable for the experienced. This is due to necessity for a shot of a sliced ball or a hooked ball and handling ability. However, it is required to further improve ball hitting sound and ball hitting feeling also in a cavity back type. In order to improve ball hitting properties, superior advantages of a muscle back type should be applied also to a cavity back type.
To this end, it is effective to increase the hardness of the face without quenching to provide a golf club which can give good shot feeling. The present applicant proposed a golf club in which a plurality of recesses are formed by forging on the rear surface of the face to improve the strength of the face in order to reduce the thickness of the face and to increase the sweet area in a golf club head (see Japanese Patent Application Laid-open No. H9-38252: Patent Document 1). The invention described in Patent Document 1 achieved to reduce the thickness of the face but failed to improve ball hitting feeling. A golf club was also proposed in which the hitting surface of the face is modified to have hardness distribution of a striped pattern along score lines in order to apply large rolling capability to a ball for long distance (e.g., see Japanese Patent Application Laid-open No. H10-108927: Patent Document 2).
In the invention described in Patent Document 2, two or more materials having different hardness are overlapped and joined by diffusion or subjected to heat treatment by irradiation of laser beam for heat treatment in a striped manner to give hardness distribution of a striped pattern to the hitting face. Since heat treatment is performed with laser beam in order to increase the hardness, composition near the surface of the hitting face is hardened but forging effects may be reduced by the heat of the laser beam. Moreover, a technique for distributing fiber flows in a face portion is known (e.g., see Japanese Patent Application Laid-open No. 2009-261908: Patent Document 3). Further, a golf club head is known in which the face surface is subjected to carburizing treatment to increase surface hardness and hardness of the face surface is differentiated in a stepwise manner in the thickness direction (e.g., see Japanese Patent Application Laid-open No. 2005-319019: Patent Document 4). Furthermore, a case is known in which the face portion is subjected to blast processing after engraving score lines (e.g., see Japanese Patent Application Laid-open No. 2004-141277: Patent Document 5).
In recent years, attention has been paid to ball hitting sound and ball hitting feeling with the change of the rule about spring-like effect (SLE). That is, comfortability of ball hitting sound and ball hitting feeling has been emphasized as well as rebound characteristics, directional property, carry, etc. In order to quantitatively evaluate the ball hitting sound and ball hitting feeling, a research for measuring bending strain of a shaft and vibration transmitted to hand has been conducted. However, there is no decisive definition as for ball hitting sound and ball hitting feeling that brings confotability. It has been attempted to give quantitative evaluation in various ways in addition to giving qualitative evaluation by players. As an example, it has been known that vibration of a shaft in circumferential direction is measured with a three-axis acceleration meter and the data is analyzed to evaluate ball hitting feeling (e.g., see Japanese Patent Application Laid-open No. 2008-125722: Patent Document 6). Also, in Patent Document 3, results of evaluation by ball hitting sound whose frequency is relative low and which leaves lingering sound are also disclosed. In any case, to increase the hardness of the face without quenching is effective for ball hitting sound and ball hitting feeling, so that a proposal concerning the relationship has been desired.